Hard problem on the chain rule for multivariable function ??

Hi ..
this is a problem from old exam ..
am lost .. i cant do anything!
i know the chain rule for F(x,y) where x=g(s,t) and y = h(s,t)
or for f(x,y,z,...) and x=g(s,t,...) y = h(s,t,....) and z=m(s,t,....) .. etc
what am gonna to said as am good !
but am lost at this

Hi ..
this is a problem from old exam ..
am lost .. i cant do anything!
i know the chain rule for F(x,y) where x=g(s,t) and y = h(s,t)
or for f(x,y,z,...) and x=g(s,t,...) y = h(s,t,....) and z=m(s,t,....) .. etc
what am gonna to said as am good !
but am lost at this